U.S. Pat. App. Pub. No. 2003/0113244A1 discloses a process for the production of a synthesis gas (syngas) mixture that is rich in carbon monoxide (CO), by converting a gas phase mixture of CO2 and H2 in the temperature range 500-600° C. in the presence of a catalyst based on zinc oxide and chromium oxide, but not including iron. The presence of both Zn and Cr was indicated to be essential for formation of carbon monoxide and hydrogen mixture at a good reaction rate, whereas the presence of Fe and/or Ni was to be avoided to suppress formation of methane via so-called methanation side-reactions. Formation of methane as a by-product is generally not desired, because its production reduces CO production. The co-production of methane may also reduce catalyst life-time by coke formation and deposition thereof.
In the past decades, numerous processes have been developed to produce syngas, which is one of the most important feedstocks in the chemical industry. Natural gas and (light) hydrocarbons are the predominant starting material for making syngas. Syngas is successfully used as synthetic fuel and also in a number of chemical processes, such as synthesis of methanol, ammonia, Fischer-Tropsch type synthesis and other olefin syntheses, hydroformylation or carbonylation reactions, reduction of iron oxides in steel production, etc.
Such syngas processes frequently use methane as the dominate feedstock, which may be converted to syngas by steam reforming, partial oxidation, CO2 reforming, or by a so-called auto-thermal reforming reaction. One of the disadvantages associated with syngas production by steam reforming of methane, which is the most widely applied process to produce syngas, is that the composition of the produced gas mixture is limited by the reaction stoichiometry to H2/CO ratios of 3 or higher.
In order to avoid such drawbacks and to help counteract increasing carbon dioxide (CO2) concentrations in the atmosphere, research has been conducted to manufacture syngas from CO2 as a raw material. The conversion is based on an equilibrium reaction shown in Equation 1 below:CO+H2O⇄CO2+H2  (1)The forward reaction is known as the water gas shift (WGS) reaction, while the reverse reaction is known as the reverse water gas shift (RWGS) reaction.
Conversion of CO2 to CO by a catalytic RWGS reaction has been recognized as a promising process for CO2 utilization, and has been the subject of various studies in the past decades. Early work proposed iron oxide/chromium oxide (chromite) catalysts for this endothermic reaction; see, e.g., U.S. Pat. No. 1,913,364. Disadvantages of these catalysts included methane formation and the accompanying catalyst coking problem.
GB2168718A discloses combining the RWGS reaction with steam reforming of methane. The combination of the two reactions allows the molar ratio of hydrogen to carbon monoxide (H2/CO) to be adjusted and to better control the stoichiometric number (SN) given by ([H2]−[CO2])/([CO]+[CO2]) in the final syngas mixture to values other than about 3 or higher, depending on the intended subsequent use of the syngas mixture.
GB2279583A discloses a catalyst for the reduction of carbon dioxide, which comprises at least one transition metal selected from Group VIII metals and Group VIa metals supported on ZnO alone, or on a composite support material containing ZnO. In order to suppress methane formation and catalyst deactivation, stoichiometric hydrogen/carbon dioxide mixtures and low reaction temperatures were used, which resulted in relatively low carbon dioxide conversion.
U.S. Pat. No. 5,346,679 discloses the reduction of carbon dioxide into carbon monoxide with hydrogen using a catalyst based on tungsten sulfide.
U.S. Pat. No. 3,479,149 discloses using crystalline aluminosilicates as catalyst in the conversion of carbon monoxide and water to carbon dioxide and hydrogen, and vice versa.
In WO1996/06064A1 a process for methanol production is described, which comprises a step of converting part of the carbon dioxide contained in a feed mixture with hydrogen to carbon monoxide, in the presence of a WGS catalyst exemplified by Zn—Cr/alumina and MoO3/alumina catalysts.
WO2005/026093A1 discloses a process for producing dimethylether (DME), which comprises a step of reacting carbon dioxide with hydrogen in a RWGS reactor to provide carbon monoxide, in the presence of a ZnO supported catalyst; a MnOx (x=1˜2) supported catalyst; an alkaline earth metal oxide supported catalyst and a NiO supported catalyst.
EP1445232A2 discloses a RWGS reaction for production of carbon monoxide by hydrogenation of carbon dioxide at temperatures of about 560° C. in the presence of a Mn—Zr oxide catalyst. A drawback of this process for syngas production as disclosed above is the selectivity of the catalyst employed; that is methane formation from carbon dioxide is still observed as a side-reaction. In the illustrative example, this methane formation was quantified as 0.8 vol % of methane being formed in the gas output of the reactor, at a degree of conversion of carbon dioxide of 40%.
GB2168718A and U.S. Pat. No. 6,328,945B1 also disclose processes that combine methane reforming and RWGS steps, but these publications do not describe or suggest the use of a catalyst as defined in the present invention.
While numerous catalysts and processes have been developed for the production of syngas from hydrogen and carbon dioxide, there is still a need in the art for new, distinct and often improved catalysts and processes for the production of usable syngas mixtures from carbon dioxide and hydrogen, where the catalysts and processes result in relatively high carbon dioxide conversions with minimal or no production of alkane (e.g. methane) byproducts and where the catalysts are stable and slow to deactivate even after extended on-stream times.